Time dependent statistics of plasma discharge parameters during bulk AC plasma electrolytic oxidation of aluminium

A method for the measurement of parameters of plasma discharge events during plasma electrolytic oxidation processing is detailed and applied to AC plasma electrolytic oxidation of aluminium. Data were obtained for processing durations from 2.5 to 40 min. Statistical distributions of plasma discharge parameters such as duration, peak current and charge transferred, are measured and presented as functions of processing duration. Event durations spanned 10–400 μs. Discharge currents were from <10 mA to more than 100 mA. Charge transfer by single events ranged from <1 μC to over 20 μC. The frequency of plasma discharges per unit time and area was determined throughout processing, falling from initial values in the range 300–350 mm−2 s−1 to fewer than 50 mm−2 s−1 after 1000 s of processing. A correlation between the upper bound of plasma electrical energy dissipation and the typical size of coating surface features (represented by coating roughness) is demonstrated. Quantitative relationships between discharge duration, peak current and the applied voltage are presented and compared to previously published data for a simpler, but less representative of coating deposition, experimental DC plasma electrolytic oxidation system. It is demonstrated that plasma electrolytic oxidation plasma discharge current is bounded by the applied voltage during processing.